1] Your paper (Markers are just more versatile. With watercolors, if you don't have the right heavyweight paper stock, there goes your art)

2] Markers are for speed. Some people can get very painterly effects with markers (like Cat Staggs), or just great graphic designs (like Nar). But they did it MUCH faster than paint would take, plus virtually no clean up.

2] Watercolors have their place. It's not really a "marker vs. watercolor" question, but rather "what's more appropriate for the job".

3] personal preference. Watercolors have a lot steeper learning curve to get into, but I feel markers ultimately have a wider range.